[MINOR] add a ->private member to the stream_interface

iohandlers will need to store some form of context and for this will
need a way to find their call context. We add the ->private as well
as ->st0 and ->st1 for that purpose. Most likely ->private will be
initialized to the current session and ->st0 and ->st1 will be used
to maintain any form of internal state between calls.

include/types/stream_interface.h

@@ -98,6 +98,8 @@ struct stream_interface {
 	struct buffer *ib, *ob; /* input and output buffers */
 	unsigned int err_type;  /* first error detected, one of SI_ET_* */
 	void *err_loc;          /* commonly the server, NULL when SI_ET_NONE */
+	void *private;          /* may be used by any function above */
+	unsigned int st0, st1;  /* may be used by any function above */
 };